The present invention relates to a protecting circuit, and more specifically to a protecting circuit of the type of ASO (Area of Safety Operation).
Output power transistors used, for example, in a transistor power amplifier circuit must be operated within an ASO region so that permanent second breakdown is not developed in the collector/emitter voltage V.sub.CE -collector current I.sub.c characteristics. If the operation of the transistor falls out of the ASO region, the transistor develops second breakdown and is permanently destroyed.
FIG. 3 shows a protecting circuit of the type of ASO for restricting the operation of the transistor within the ASO region to protect the transistor from permanently destroyed, as has been disclosed, for example, in Japanese Utility Model Laid-Open No. 51-101752. The operating condition of an output transistor Q.sub.4 is detected by a detector circuit made up of resistors R.sub.1, R.sub.2 and R.sub.3. The detection output of the detector circuit is applied to the base of a control transistor Q.sub.1 which controls a base current of the output transistor Q.sub.4. In case the operation of the output transistor Q.sub.4 is likely to fall outside the ASO region, the control transistor Q.sub.1 is rendered conductive thereby to limit the base current of the output transistor Q.sub.4. According to the above prior art, however, the forward voltage V.sub.BE at the base-emitter junction of the control transistor Q.sub.1 has been set to be equal to a threshold voltage at which the protecting circuit commences the protecting operation. Therefore, there arise such detects that the level of protecting operation is varied since the junction voltage across the base and emitter is dependent upon the temperature, or the level at which the protecting operation commences is greatly dispersed due to the dispersion in current amplification factor h.sub.FE of the control transistor Q.sub.2 or due to the dispersion in the base layer resistance r.sub.bb.
Japanese Patent Laid-Open No. 50-81247 discloses another protecting circuit of the ASO type as diagramatized in FIG. 4.
As has been disclosed in the above Laid-Open specification, a collector current I of an output transistor Q.sub.4 of the circuit is found from the following equation (1), ##EQU1##
In this circuit, a voltage V.sub.BEQD across the base and emitter of a transistor Q.sub.D which equivalently operates as a diode when a circuit of collector and base is short-circuited, is smaller than a voltage V.sub.CE across the collector and emitter of the output transistor Q.sub.4. Therefore, the voltage V.sub.BEQD is negligible. Thus, it is possible to reduce the effects upon the dispersion of element constants or upon the change in temperature (especially, it is possible to reduce the effects caused by the temperature dependency of the forward voltage at the base/emitter junction of the transistors).
According to the abovementioned prior art, however, since the power-limiting equation (1) assumes a curve of exponential function, it becomes very difficult to design the circuit which corresponds to a curve of allowable power loss of the collector of the output transistor Q.sub.4 or which corresponds to the ASO region.
Through the research conducted by the inventors of the present invention, on the other hand, it become obvious that the power-limiting equation (1) which describes a curve of exponential function is disadvantageous because of the reasons mentioned below.
In a push-pull output amplifier circuit which operates on a power-supply voltage V.sub.cc of positive polarity to drive a load resistance R.sub.L via an output coupling capacitor, the two output transistors which are connected in series between the power-supply voltage V.sub.cc and a ground potential GND are usually biased in a d-c manner such that the voltage across the collector and emitter of each of the transistors is about V.sub.cc /2. Therefore, when an output voltage swing is raised and lowered in response to the input signals with the d-c operation level of V.sub.cc /2 as a center, and when the input signals do not cause the voltage to rise to the power-supply voltage V.sub.cc and do not, either, cause the voltage to lower to the ground potential GND, the output of the push-pull amplifier circuit acquires a non-clipped state. Under the non-clipped output condition, the locus of operation determined by the collector/emitter voltage V.sub.CE -collector current I characteristics of a single output transistor does not exceed a straight line connecting the two operation points [O(V), V.sub.cc.2R.sub.L (A)] and [V.sub.CC (V), O(A)].
On the other hand, when excessive input signals are applied, the two output transistors are alternately rendered conductive and nonconductive responsive to the input signals. Therefore, the output of the push-pull output amplifier circuit is raised up to the power-supply voltage V.sub.cc and is lowered to the ground potential GND responsive to the input signals, thereby to produce pulse waveforms clipped between the upper and lower levels. Under the clipped condition, the two transistors perform switching operations between the two levels, i.e., between the power-supply voltage V.sub.cc and the ground potential GND. However, the locus of operation of a single output transistor under the clipped output condition caused by excessive input signals, does not exceed a straight line connecting the two operation points [O(V), V.sub.cc /R.sub.L (A)] and [V.sub.cc (V), O(A)].
Particularly, with the audio output amplifier having small outputs (10 watts to 20 watts), it is desired not to operate the protecting circuit even under clipped output conditions caused by excessive inputs. Namely, when the protecting circuit is operated, a negative feedback loop is established to connect the output and the input of the output transistor, giving rise to the occurrence of oscillation phenomenon or causing the output to be extremely distorted.
In the audio output amplifiers, therefore, it is desired that the ASO characteristics lie between maximum allowable power loss characteristics (hyperbola) of the collector of the output transistor and the abovementioned operation locus (straight line) under the clipped output condition. Therefore, according to the prior art as shown in FIG. 4, the power-limiting equation (1) describes a curve of exponential function, causing the design of the circuit to be complicated.
Further, referring to the power-limiting equation (1), even when a resistor R.sub.2 ' is constituted by an equivalent resistance of an aluminum wiring layer in a monolithic semiconductive integrated circuit to have a resistance of about 20 m.OMEGA., the resistance of the resistor R.sub.1 ' must be set to a considerably great value in order to obtain a limiting current of about 2 amperes. Hence, there remains such a problem that the dispersion in the resistance of the resistor R.sub.1 greatly affects the limiting current.
In order to preclude the abovementioned problems, the present invention provides a protecting circuit of the ASO type.